1. Field of the Invention
The present invention relates to touch panels for operating electronic devices.
2. Background Art
With electronic devices such as mobile phones and car navigation systems becoming increasingly sophisticated and diversified, many products are adopting a light-transmissive touch panel on a front face of a display device, typically a liquid crystal display. The user of the electronic device views what is displayed on the display device on a rear face of the touch panel through this touch panel, and presses the touch panel typically with a finger or pen so as to operate it and switch between the functions of the electronic device. Accordingly, the user requires a touch panel with good viewability and reliable operation.
A conventional touch panel is described next with reference to FIG. 4.
FIG. 4 is a sectional view of the conventional touch panel. In FIG. 4, wave plate 101 is a quarter-wave plate which is given birefringence by stretching a film typically of polycarbonate resin. Lower substrate 102 is light-transmissive. Light-transmissive upper conductive layer 103, made typically of indium tin oxide, is formed at a lower face of wave plate 101. Light-transmissive lower conductive layer 104, made typically of indium tin oxide, is formed at a top face of lower substrate 102.
A plurality of dot spacers (not illustrated) are formed, using insulting resin, at predetermined intervals on a top face of lower conductive layer 104. A pair of upper electrodes (not illustrated) is formed on both ends of upper conductive layer 103. A pair of lower electrodes (not illustrated) is formed on both ends of lower conductive layer 104 in a direction perpendicular to the upper electrodes.
An adhesive layer (not illustrated) is applied to top and lower faces of frame-like spacer 105. This adhesive layer attaches wave plate 101 and lower substrate 102 together by their outer borders. Accordingly, upper conductive layer 103 and lower conductive layer 104 are disposed facing each other with a predetermined space in between.
Polarizing plate 106 is formed typically by laminating triacethyl cellulose film on top and lower faces of a stretched and oriented polarizer to which iodine and dye have been adsorbed in polyvinyl alcohol. This polarizing plate 106 is attached to a top face of wave plate 101 to configure touch panel 100.
Touch panel 100 is disposed at a front face of typically a liquid crystal display device (not illustrated), and then mounted on an electronic device (not illustrated). A pair of upper electrodes and a pair of lower electrodes are coupled to electronic circuit (not illustrated) of the electronic device.
A top face of polarizing plate 106 at central part 106a, end 106b, and so on is pressed typically by a finger or pen while the user views what is displayed on the liquid crystal display device on a rear face of touch panel 100. For example, touch panel 100 is operated in a direction indicated by arrow E or arrow F. This operation makes polarizing plate 106 and wave plate 101 dent, and upper conductive layer 103 and lower conductive layer 104 come into local contact at a portion pressed.
Then, the electronic circuit applies voltage sequentially to the upper electrodes and the lower electrodes. The electronic circuit detects a portion pressed based on the voltage ratio of the upper electrodes and the voltage ratio of the lower electrodes. This switches between the various functions of the electronic device.
External light, such as sunlight or lamplight irradiated from above touch panel 100, first passes through polarizing plate 106. When the external light passes through polarizing plate 106, light in the X direction and light in the Y direction perpendicular to the X direction are linearly polarized only to either direction by polarizing plate 106. For example, when polarizing plate 106 has the characteristic of absorbing light in the Y direction, only light of linearly polarized light in the X direction pass through polarizing plate 106. This light exits from polarizing plate 106, and enters wave plate 101.
The light entering wave plate 101 is polarized from linearly polarized light to circularly polarized light as a result of passing through wave plate 101, and then exits from wave plate 101. Light exiting from wave plate 101 are reflected upward on lower conductive layer 104.
The light reflected on the surface of lower conductive layer 104 passes through wave plate 101 again. The light is now linearly polarized in the Y direction which is shifted by a half-wavelength, and enters polarizing plate 106. However, since polarizing plate 106 has the characteristic of passing through only the light in the X direction, polarizing plate 106 blocks any reflected light which is linearly polarized in the Y direction.
In other words, any external light entering touch panel 100 from above touch panel 100 is reflected upward on lower conductive layer 104. However, polarizing plate 106 blocks the light reflected on lower conductive layer 104, and thus this light does not exit from the top face of polarizing plate 106, which is the operation face. Accordingly, reflection of external light on the surface of touch panel 100 is eliminated, offering good viewability. The liquid crystal display device on the rear face of touch panel 100 is thus easily viewable.
Conventional touch panel 100 is typically disclosed in Japanese Patent Unexamined Publication No. 2000-10732.